The present invention relates to a composite roll ring, namely, a one-piece composite ring having a cast iron portion and a cemented carbide portion with a metallurgical bond therebetween, said cast iron having a carbon equivalent of from 2.5 to 6.0 and a microstructure predominantly of bainite, at least some of the bainite having been formed by the heat treatment of austenite. The roll ring may be mounted on a spindle with driving devices for transmitting torque from the spindle to the roll ring being located in the cast iron portion of the ring. In addition, methods for making the roll ring and a roll including at least one roll ring are disclosed.
The use of roll rings made of cemented carbide for hot or cold rolling has been hampered by the problem of the transmittal of torque from the driving spindle to the carbide roll rings without causing serious tensile stresses. Cemented carbides are brittle materials with limited tensile strengths and especially high notch sensitivity in inner corners such as keyway bottoms or other driving grooves, or at roots of driving lugs which are integral with the carbide ring. Use of such cemented carbide roll rings using conventional joints have proved unsatisfactory.
Another method proposed for the transmission of torque is by means of frictional forces at the bore surface of the carbide ring. However, radial force on the surface gives rise to tangential tensile stresses in the carbide rings with the maximum tensile stresses at the inner diameter. These tensile stresses are superimposed on other tensile stresses generated when the roll is in use generally leading to tensile stresses which are too high.
U.S. Pat. Nos. 3,787,943 and 3,807,012 disclose a method of making a composite roller for hot and cold rolling and the roller itself in which a ring of cemented carbide has a ferrous alloy such as steel cast about it. The composite ring is cooled such that the ferrous metal hub shrinks more than the cemented carbide ring thereby exerting compressive forces on the cemented carbide ring to hold it in place. Holes can be drilled in the hub so that an epoxy based resin can be inserted into the composite ring after shrinkage filling the holes formed by the shrinkage. No bonding between the cemented carbide ring and the ferrous body is disclosed.
However, during cooling from the casting temperature, the casing shrinks more than the carbide ring, hereby giving rise to inwardly directing forces on the carbide ring. These forces produce axially directed tensile stresses on the outer surface of a carbide ring, which tensile stresses act perpendicularly to microcracks generated in the roll surface during rolling. Under the influence of these perpendicularly directed tensile stresses, the microcracks propagate in depth which may cause roll breakage or the need for excessive dressing amounts. Both limit the total rolling capacity of the roll.
It is also known as disclosed in U.S. Pat. No. 3,609,849 to form composite roll rings which consist of a working part of cemented carbide in a casing of various metal or metal alloy powders which are then sintered about the carbide.
In this case, the casing materials are characterized either by low hardness or low yield strength. Otherwise, a cemented carbide, a brittle material, is used. Neither of these materials are particularly suitable for use in the necessary torque transmission couplings.